For good starting and warmup behavior at low temperatures, diesel engines require a heat source that preheats either the gas mixture, the intake air, or the combustion chamber. For engines in passenger vehicles, as a rule the use of sheathed-element glow plugs is recommended. These are made up of a plug shell and a glow element that protrudes from the glow shell and that, in its assembled state, extends into the combustion chamber of an internal combustion engine. As a rule, the glow elements of the sheathed-element glow plugs extend 4 mm into the combustion chamber of the internal combustion engine, and heat the diesel fuel-air mixture. The glow temperature and post-glow time of the glow element has a significant influence on the exhaust gas behavior, the fuel consumption of the engine, and the idling behavior after starting.
In order to achieve the goals of conserving more fuel and reducing emissions, there is an increasing interest in developing economical sensors that are able to supply information about the combustion behavior of the engine directly from the engine's combustion chamber. Based on this information, it could be possible, for example, to regulate the injected fuel quantity. Monitoring the pressure inside the combustion chamber would have significant advantages in comparison with, for example, ionic current measuring, which supplies only local information, because the pressure measurement values, or the changes in these values, are larger and are therefore easier to acquire.
Integrated sensor designs, in which a pressure sensor is situated on or in the sheathed-element glow plug, have the advantage that it is not necessary to provide an additional bored hole in the internal combustion engine. This advantage is all the greater in modern internal combustion engines, in which the constructive space for mounting additional sensors is very limited.
In the prior art, solutions have been proposed in which a piezoelectric element is provided between the housing and the screw connection on the cylinder of a sheathed-element glow plug, as known for example from EP A 1 096 141 A. In addition, in the document WO 97 09 567 A, an additional pressure sensor in the sheathed-element glow plug is proposed that is connected intermediately, between a fixing element and an element for receiving cylinder pressure, in a body of a sheathed-element glow plug.
According to the prior art, in sheathed-element glow plugs glow elements are pressed together fixedly with the plug shell, in order to ensure a good ground coupling and sealing.
A sensor mounted inside the sheathed-element glow plug has the task of acquiring the minimum change of length (which is in the μm range) when high pressure is applied; a piezosensor can be used for this purpose.